BC Guide: Finding the right bottom bracket for your bike

The bottom bracket is literally a central component on every bicycle. Only when the bearings rotate smoothly can you generate propulsion via the pedals and cranks. However, this component, which is itself quite simple, comes in so many variants and dimensions that it is easy to lose track of everything. The term "bottom bracket" comes from the fact that in the past most bottom brackets were placed in the frame. Today, the bearings are often located in bearing cups outside the bottom bracket shell. The bottom bracket takes on the difficult task of mediating between frame and cranks, which don’t usually come from the same manufacturer. The list of interfaces to the frame on the one hand is diverse – whether threaded or not. On the other hand, it is important to consider the compatibility with the crank. Whether you want to replace a worn-out bearing or build a custom bike, our guide will help you find the right bottom bracket in just a few steps.

The Shell Width

The bottom bracket shell – the part of your frame in which the bottom bracket is installed – has a certain width. It is relatively easy to measure, often even while a bottom bracket is installed. You only have to measure the frame from right to left, without the bearing cups or spacers. With the help of spacers, some bottom brackets can also be adapted to different shell widths. The filters will usually take this into account. Once you have selected the shell width, the number of other filters is reduced to a more manageable level.

Assembly Types: Threaded Bottom Brackets

Bottom brackets for threaded shells can be recognised during assembly by the fact that the inner or outer bearing cups have some kind of gearing that allows them to be gripped and screwed in with a tool. They are currently only available in three different versions, which you can distinguish relatively easily by the inner diameter of the bottom bracket shell and the direction of the threading when the bearings are removed:

The Axle Type

The “axle type” filter lets you narrow down the selection of bottom brackets according to crank compatibility. You have the option of measuring your crank yourself or obtaining the parameters from the data sheet or the item’s description. We can sort axle types into two groups: systems where the axle is part of the crank (often: two-piece cranks) and systems where the crank axle is part of the bottom bracket. There is not a bottom bracket for every crank and frame combination. In some cases, however, bottom brackets can themselves act as adapters to mediate between systems or can be installed with the help of adapters.

Axle Types as Part of the Bottom Bracket

On older or cheaper bikes you will often find systems where the crank axle is part of the bottom bracket. These are usually sealed cartridges, i.e. self-contained units that disappear almost completely into the frame. The two crank arms are individually screwed onto the axle. To transmit forces, the interface between the crank arm and axle is either a tapered square profile or a geared profile. The usual systems are:

• Square (JIS) – found predominantly among Asian manufacturers (e.g. Shimano)
• Square (ISO) – found predominantly among European manufacturers
• Octalink MTB – multi-tooth with eight teeth, gearing longer than with Octalink Road
• Octalink Road – multi-tooth with eight teeth, gearing shorter than Octalink MTB
• ISIS – multi-tooth with ten teeth
• Power Spline – multi-tooth with twelve teeth

The two square variants are not compatible with each other. Having the wrong bottom bracket could damage your crank. Since they are hard to differentiate, you should check with the manufacturer of your crank to find out which square fits. In addition to the installation width of the bottom bracket itself (see above), the length of axle must be selected to match the crank. It determines the chainline, the Q-factor and ensures that your crank does not collide with the chainstays. You can find out which axle length is required for your crank either in the manufacturer’s documentation or by measuring the old axle with the crank disassembled.

Bearing Type and Quality

Varying price points of compatible bottom brackets usually differ in terms of manufacturing quality and the type of bearings used. Nowadays, encapsulated ball bearings are mostly used – often referred to as cartridge bearings. In some cases, you can replace them individually when they wear out. This saves time and money. Please also follow the manufacturer's care instructions! You can lubricate and adjust some bottom brackets yourself to get the best performance and durability. Rolling element bearings are made of either classic bearing steel, stainless steel or ceramic. While normal bearing steel is very durable when well cared-for, stainless steel has the advantage of being more resistant to corrosion (the simplified term is rust-free). If you ride a lot in the rain or wash your bike often and don’t have time to inspect and re-grease the bearings regularly, stainless steel is a good choice. Ceramic bearings are also corrosion-resistant, lighter and particularly smooth-running.